For fast, reliable furnace repairs, call Georgia Air Cooling & Heating at 912.513.3361 today!
When turning on your heat, have you ever wondered why the hot air doesn’t immediately release into different areas of your home? Because your furnace produces rather than stores warmth, it takes some time for the existing cooler air to pass through the system and heat up with the burner before expelling back into your surrounding air. Still, it takes more than a sound thermostat to monitor air temperature.
A furnace limit switch allows air to move within your heating system, but what does a furnace limit switch do that makes it the brains of your unit? Our #1 team at Georgia Air Cooling & Heating, Richmond Hill’s furnace replacement and installation experts, is here to break it down. Below, we’ll delve into this vital component and what you can do to keep it working optimally.
How Does a Furnace Limit Switch Function?
When temperatures drop below your thermostat setting, the thermostat sends signals to the igniter to power on, which then effectively lights the combustion chamber fuel. After the furnace burners heat the natural gas, the fan blows the air over the heat exchanger. It then passes the air through the plenum boxes on either side of the heat exchanger, which connects to the furnace supply outlet.
Meanwhile, the limit switch below the supply plenum notes the changing temperature to decipher when to tell the air handlers to distribute the air into your home. Throughout the heating cycle, the limit switch monitors the temperature in the furnace, preparing to shut down if it gets too hot so the unit can cool down. The limit switch also keeps cool or mildly warm air from releasing prematurely.
What Causes Limit Switch Malfunctions?
With constant functioning, your limit switch wears out like any other component, needing regular servicing and repairs when necessary. However, you should know the key signs of a faulty limit switch so you can call for professional assistance before the problem worsens, jeopardizing your unit and air quality.
Dirt and grime buildup on the limit switch sensor keeps the flame sensor rod from detecting the surrounding temperatures, causing it to trip. A professional has to locate the limit switch behind the access panel, unscrew it from the plenum, and gently clean it with steel wool to remove all stuck-on debris. However, other factors cause the switch to react negatively, keeping it from releasing optimal air.
Other Factors that Affect Limit Switch Functionality
If your furnace is short cycling, turning on and off before completing one full heating cycle and leaving your home with hot and cool spots, there may be an overheating issue. Overheating occurs for numerous reasons, from dirty furnace filters blocking airflow to blower motor issues preventing quality air from circulating. Despite the cause, excessive heating trips the switch, shutting down the unit.
Similarly, a faulty thermostat dictates incorrect temperatures to your system. If it falsely reads your indoor climate higher than it actually is, your limit switch sends signals to your air handlers to stop dispersing air before effectively warming your home. If it reads it lower, your system may long cycle or overheat, causing the switch to power the gas supply off abruptly to prevent dangerous temperatures.
Other part failures that place excessive wear on the limit switch include:
- A defective gas valve not bringing in fuel for heating
- The blower fan not sucking air into the system
- Fried circuits or a faulty circuit board failing to control the heating cycle
What Happens If Your Limit Switch Becomes Faulty?
Now that we have the answer to “what does a furnace limit switch do?” what happens when it doesn’t have the chance to do what it’s supposed to? A dead limit switch won’t power on the air handlers, keeping air from circulating and exiting your unit. That means in the low 40-degree temps of Richmond Hill winters, you won’t feel the relief of a warmer indoor climate, keeping the temperature frigid.
If your limit switch is still functioning, but not optimally, your furnace may still turn on. However, your furnace may blow lukewarm or cool air upon startup since the limit switch isn’t preventing the air from escaping before properly heating up. Since the switch is one of the system’s safety features, a failing one also pushes your system to run continuously without shutting off to keep it from overheating.
With an overheating unit, you’ll notice:
- Unusual banging, grinding, or screeching from failing parts or a cranked-out blower motor that cannot release heat properly
- A burning smell from parts wearing down or grinding together as they come loose
- A complete breakdown that won’t allow your system to power on at all, sometimes requiring a full unit replacement rather than a repair
How to Keep Your Limit Switch in Proper Working Condition
The best way to ensure your system is safe throughout its lifespan is by keeping up with at-home and semi-annual professional maintenance.
Every three to four months, clean or discard your air filters and replace them with fresh ones to encourage optimal airflow. Also, wipe down your air duct and furnace vents to keep dust, dander, and other contaminants from entering your system and blocking circulation. This process also keeps grime from accumulating on the limit switch, which would otherwise obstruct its temperature sensors.
If you notice any of the above concerns pointing to a tripping limit switch, leading your system constantly into lockdown mode, shut off the power to your system, remove the access panel, and look for the limit switch. It should look like a yellow or red button or switch near the ignition that you flip or push to reset the furnace. Once reset, reinstall the access panel and turn on the electricity again.
If these shutdowns occur regularly or you don’t know how to find the limit switch, call an experienced HVAC technician. They’ll take apart your entire system to confirm whether the limit switch is the reason for your unit’s malfunctions. The professional team should also analyze the level of airflow and if any other component issues are causing the furnace switch to shut off too soon or prolong heating cycles.
They may swap out your filters and clean hard-to-reach components within your unit to prevent overheating from dirt. Then, they’ll check your thermostat’s accuracy, battery level, and wiring after replacing broken parts and tightening loose pieces. If a steel wool cleaning isn’t enough for your limit switch, the technicians relay the best replacement options from top manufacturers.
Testing the Limit Switch
Your HVAC technician knows more than just the answer to “what does a furnace limit switch do?” They understand how it responds to unusual behavior in your unit and how to conduct a continuity test to determine if a new limit switch is vital.
The technician disconnects the power from the switch before testing it by either detaching the terminal connectors or turning your electricity off so that external power sources don’t interfere with the reading. Then, using a multimeter, they send a spark of electrical current between each switch terminal, which is where the switch connects to circuits.
However, before doing so, they’ll test the multimeter by touching the probes to one another. A zero reading means the device’s calibration and functionality are up to par. They’ll then use this current to measure the resistance between terminals in ohms.
If the reading shows a zero or near zero ohms, that indicates a closed switch, which is functioning properly to allow current to flow through it. However, a reading of 0.2 ohms or higher or an “OL” display gives off an open switch error code. This error usually occurs when the furnace overheats or the switch goes bad, and in the case of the latter, your technician replaces the furnace limit switch.
For Peachy Georgia Air Indoors Year-Round!
A limit switch is just one of the many components that make up your furnace. Yet, the moment it shows signs of wear and tear, your entire unit and family’s safety could be in jeopardy. Rather than have yours overheat when you need it most, causing it to power down indefinitely, call on the number one Georgia HVAC team in the business.
At Georgia Air Cooling & Heating, our Trane comfort specialists dedicate themselves to creating absolute comfort year-round in every residential and commercial environment. With 100% customer satisfaction due to our superior services, high-quality products and tools, and our price match guarantee, you can’t afford not to call us the moment your furnace is on the fritz.
We conduct scheduled furnace maintenance, repairs, and replacements, alongside emergency furnace repair help, any time of the day. We guarantee complete satisfaction when providing you with a new unit and will even pay you $300 with a free repair if something goes wrong within the first two years.
When you need to know “what does a furnace limit switch do?” or require assistance, call Georgia Air Cooling & Heating at 912.513.3361. We’re the clear choice in Richmond Hill and the surrounding areas!